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Not worst case Greenland's glaciers are slipping into the ocean at an ever increasing rate, but their impact on sea levels may be less than previously predicted, a ten-year study shows.

The finding means that global sea levels aren't likely to rise up to two metres by 2100, the worst case scenario proposed by previous studies, which also suggested Greenland would contribute half a metre sea level rise from melted ice sheets.

"It looks like we aren't heading towards a worst case scenario," says Twila Moon, glaciologist at the University of Washington, and lead author of the study published today in Science.

Moon explains that the velocity of glaciers provides information about future ice loss. The faster the glaciers move, the quicker they reach the ocean, break off icebergs and melt.

In this study scientists used satellites to measure the velocity of almost 200 Greenland glaciers from 2000 to 2010. This is the first time scientists have measured such a large number of glaciers on this landmass over a prolonged time period.

Their results showed that the velocity of individual glaciers varied. Some moved quickly - at around 10 kilometres per year - while others slowed down. Although local temperatures and topography influenced glacier behaviour, scientists observed an overall increase in glacier velocity of about 30 per cent over the last decade.

"The glacier speed-up is real, it's significant, but it's much more gradual than we had expected," says Dr Ian Joughin, glaciologist at the University of Washington and co-author of the paper.

Future predictions

Joughin says that Greenland's ice sheets are likely to contribute about 10 centimetres towards future sea levels by 2100. He also explains that many other sources contribute to sea level rise; the ocean is warming and expanding, and Antarctic and mountain glaciers are also melting.

"All these sources combined may cause a sea level rise of 50 centimetres to 1 metre by the end of the century," says Joughin. "This would cause increased flooding in low lying areas such as South Florida, Amsterdam and Bangladesh."

Dr John Church, oceanographer at the Centre for Australian Weather and Climate Research in Hobart, Tasmania, describes the study as comprehensive and agrees with the findings.

"While this paper says there has been an acceleration in glacier speeds in Greenland, it's not of a large enough magnitude to cause the two metre sea level rise that has been postulated by some scientists," he says.